Posts tagged 'polygons'

F.15 Cyclic Quadrilateral

A cyclic quadrilateral WXYZ whose interior angles form the arithmetic progression theta, 2*theta, 3*theta, 4*theta has theta = 36 degrees (W = 36, X = 72, Y = 144, Z = 108) and golden-ratio diagonals |XZ| = 2 sin(36) = sqrt(3 - phi) and |WY| = 2 sin(72) = phi*sqrt(3 - phi) in the unit circle; by contrast, no non-trivial cyclic quadrilateral can have interior angles in geometric progression theta, k*theta, k^2*theta, k^3*theta.

F.40 Regular Polygons and Nested Radicals

For a regular N-gon inscribed in a unit circle (R = 1), doubling the number of sides maps edge length E_{N/2} to E_N = sqrt(2 - sqrt(4 - E_{N/2}^2)), generating closed-form nested quadratic radicals for the 2^k, 3*2^k, 5*2^k, and 15*2^k polygon families.

F.45 Squares, Subdivided Lattices, and the Japanese Theorem

A square inscribed in a circle of radius R has side s = sqrt(2) R; inside a square of side s, intersecting cevians divide the interior in exact rational proportions, and by Carnot’s Theorem the sum of inradii in any triangulation of a cyclic polygon is invariant.